How would you hide the cracks in cell fracture?
You could put the unsharded cube in one layer and the shards in another layer:


by selecting the shards
using M and click the other layer


(the shards are already put in another layer by default when using the cell fracture)


Then in the properties panel, in the renderlayers tab you could use keyframes in the exclude section (assuming you use cycles render).
Before collision, keyframe in such a way that only the shards-layer is active in the exclude section. After collision, keyframe in such a way that only the unsharded-layer is active in the exclude section. Make sure in the scene section both the layers are active.


But using this exclude method, just must make sure the unsharded cube and shard move in a same way for smooth transition during switching excluded layers

For example, if the collision occurs at frame 100. Then at frame 100, it looks like this:

And at frame 101:


To make the cube break more realistically, you could use the Bullet constraint tool https://www.dropbox.com/s/fq3nmhl4b6yyhc3/Bullet_Viewport_Constraints_Tool.zip
Select shards, define your Search Radius (100 works ok) and Neighbour limit (10 works ok), and turn on breakable (enable checkbox twice!) and define the breakable value (higher value make it less easy to break).

After you make changes in these variables (while shards selected), then click 'update selected'